Autotrophic nitrifying bacteria are of considerable ecological importance in soils and in the oxygenated zone of sediments. So far, long-term experiments (usually 21-28 days) on the influence of chemicals on nitrification in soils have been carried out by incubation of soil ammended with ammonia and the determination of ammonia, nitrite and nitrate at the end of the incubation period. However, the method has several drawbacks and the results of these experiments are often difficult to interpret. In the present study, the influence of dicyandiamide and anthracene oil on the first step of soil nitrification, e.g. autotrophic ammonia-oxidation, has been determined in long-term experiments. By regular sampling of soils treated with the test substances, changes in the actual and potential ammonia-oxidation rates were determined using the chlorate inhibition technique. Additionally, the actual and potential respiration rates were estimated.